1. Technical Field
The present invention relates in general to an improved hard disk drive and, in particular, to an improved apparatus and method of providing a limit stop having both bilinear and nonlinear properties for a hard disk drive actuator.
2. Description of the Related Art
Generally, a data access and storage system consists of one or more storage devices that store data on magnetic or optical storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The hard disks themselves are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating. Typically, one to six disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm).
A typical HDD also utilizes an actuator assembly. The actuator moves magnetic read/write heads to the desired location on the rotating disk so as to write information to or read data from that location. Within most HDDs, the magnetic read/write head is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk.
Typically, a slider is formed with an aerodynamic pattern of protrusions on its air bearing surface (ABS) that enables the slider to fly at a constant height close to the disk during operation of the disk drive. A slider is associated with each side of each disk and flies just over the disk's surface. Each slider is mounted on a suspension to form a head gimbal assembly (HGA). The HGA is then attached to an actuator arm that supports the entire head flying unit. Several arms may be combined to form a single movable unit having either a linear bearing or a rotary pivotal bearing system.
The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop and settle directly over the desired track.
In hard disk drives, limit stops (LS) are typically used to safely limit the motion of the actuator when it reaches the extreme ends of its stroke. The LS is usually a pin or bumper that is contacted by the arms of the coil yoke of the head-stack assembly (HSA). When the LS is contacted by the yoke arm of the HSA, the kinetic energy of the HSA is converted into strain energy by deforming the LS, and also into heat by virtue of the material damping of the LS. The design of the LS encompasses three main requirements. First, the LS should have good locational accuracy. Second, the LS should have an appropriate stiffness for absorbing the kinetic energy of the HSA in the space that is allotted for limit stop travel. Third, the LS should have high damping so that the velocity of the HSA when leaving the limit stop is as small as possible.
In recent hard disk drives (HDD), there are several common designs for the LS. A typical design uses a molded polymer collar or bumper that is pressed over a pin or peg that is either cast or pressed into the base casting of the HDD. Other LS designs incorporate shapes without a central axis (e.g., rectangular) that are located to the base casting by a pin or hole. These designs provide good locational accuracy and good damping characteristics, according to the damping properties of the polymer selected. Typically, the stiffness versus compression curve for the LS is non-linear, due to both the material properties of the polymer used, and the contact condition between the yoke arm of the HSA and the LS. One detrimental effect of current designs is that the yoke arm may stick to the LS due to the material properties of the polymer.
The advent of Self Servo Write (SSW) has created new demands on LS designs. The SSW process is a procedure that is used to write the servo tracks on the disk without the use of an external encoder, as is typically used in the traditional servo track write process. In brief, the SSW propagates the servo tracks across the disk by servoing on the edge of the previously written track, and then writes the next track.
For example, commonly assigned U.S. Pat. No. 5,612,833 describes one type of SSW system. Head positioning is achieved by first writing a reference track, then moving the head to a next position while reading the reference track until it is determined that the amplitude of the readback signal has been reduced by a predetermined amount. The determination is made on a sector-by-sector basis in a two-step process. First, the signal amplitude of each burst is compared with a corresponding normalization value measured in the same circumferential position of the last written track to obtain a propagation burst fractional amplitude. This current value is then compared to a reference value for the sector, and the difference is used as a position error signal (PES) for making corrections to the head position. The PES is also stored for later use. The normalization values are updated for each newly written track in a normalization revolution. Updating for every track has been performed previously because the propagation burst amplitudes from track to track which provide the normalization values tend to vary due, e.g., to fly height variation and modulation of the magnetic properties of the disk or other causes. New reference values are also calculated for each track during the normalization revolution and incorporate the stored PES values and have the effect of reducing track shape error growth. The new reference values each comprise a nominal reference value plus a corrective value calculated from the previously stored PES for each sector. In addition, the servo loop is designed to have a closed loop response, which causes track shape errors to decay, rather than grow, from one track to the next.
Due to magnetics design considerations and the skew of the read-write head at the extremes of the stroke of the HSA, there is a radial gap between the read and write elements of the slider. Since the SSW process needs to write the new track immediately adjacent to the previous track, the SSW algorithm needs some method to write a number of tracks to span the gap between the read and write elements at the beginning of the process.
One method uses the inner diameter LS (relative to the disks) as part of its start-up algorithm. In order to write the initial tracks, the inner diameter LS is compressed by torque resulting from electrical current applied to the VCM of the HSA. The first track is written, the current is reduced, a second track is written, and so on, until the gap between the read and write elements has been spanned. For this start-up phase, the spacing between the tracks is determined by the stiffness of the LS and the electrical current that is sent to the VCM. It is clear that the non-linear characteristics of the common LS design, which may also depend on temperature, are not suited for this purpose. Furthermore, the materials used for high damping LS often have a sticky surface that makes disengagement of the actuator from the LS quite unpredictable. This condition also causes problems for the SSW process. Thus, an improved design for limit stops in a hard disk drive, particularly those employing the SSW process, would be highly desirable.